Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

Подождите немного. Документ загружается.

Pale Ale Malt

0019 Pale ale malts are used to produce traditional, top

fermented ales. In the past, this market had been

restricted to the UK, but with the advent of micro-

breweries, demand for pale ale malts has expanded

to other parts of the world. The major difference

between pale ale and pale lager malts is the darker

colors and stronger flavors of the ale malts. This is

achieved by using a more modified malt and higher

final kilning temperatures (90



C). The higher

temperatures result in a stronger malty flavor and a

deeper color, but they also reduce enzyme levels,

resulting in more unfermentable dextrins and thus

greater body in the final beer. Mild ale malts are less

modified and are kilned at higher temperatures than

pale ale malts. The result is lower levels of extract and

enzymes but a greater production of color and flavor

compounds. Beers made from mild ale malts are

sweeter with more color and flavor.

Vienna Malt

0020 Vienna malt is a lager type malt characterized by a

red/copper color and a faint toffee flavor. This malt is

the basic ingredient of the more highly colored lager

beers, such as Ma

rzen. It is also used, in minor

amounts, to correct the color of beers made from

excessively pale malts. The malt is produced by com-

pleting the kilning of a typical pale larger malt at a

raised temperature (90



C). Also, the use of recircu-

lated air is increased at lower kiln temperatures in

order to help produce the amino acids and free sugars

required for color development at the higher final

kilning temperature.

Munich Malt

0021Munich malt is characterized by a rich aroma and

dark color. By tradition, it is used to make the dark,

aromatic, full-bodied lager beers of Bavaria. The malt

brings a more intense flavor and color to the beer than

does a Vienna malt. Munich malt is produced from

two-rowed barley with above average protein. The

malt should be slightly overmodified during germin-

ation and then allowed to stew during the first stage

of kilning. Stewing is intensified germination that

results from a greater use of moist, recirculated air

in conjunction with somewhat elevated temperatures.

This releases the amino acids and free sugars that

develop, during higher final kilning temperatures

(100–105



C), into Maillard products that are

known for their intense color and flavor. Maillard

products, such as melanoidins, also help to improve

the flavor stability of beers. Brewers strive for flavor

stability by limiting oxygen levels in their beer. The

increased reducing power of Maillard products helps

scavenge oxygen in the beer. Reasonable enzyme

activity is preserved in Munich malt, despite extreme

kiln temperatures, by drying the malt slowly.

Crystal and Carapils Malts

0022Crystal (caramel malt) and carapils malts are unlike

other colored malts in that fully modified green malts

are roasted in a sealed roaster (65–70



C) prior to

drying. As a result, the starches of crystal malts are

degraded to simple sugars during this early stewing

stage of kilning, in contrast to standard malts where

the majority of starch conversion occurs later in the

brewhouse. After this liquefaction of the endosperm,

tbl0003 Table 3 Quality characteristics and primary functions of malt types used in brewing

Malttype Type of beer Primary use Fine extract

(%)

Protein

(%)

Color

(



Lovibond)

Diastatic

power

(



Lintner)

Pale lager malt Lager Fermentable sugars and enzymes 81 10.5 1.5 85

Pale ale malt Ale Fermentable sugars and flavors 82 10.0 3.0 45

Vienna malt Lagers, bocks, various Fermentable sugars and color 81 11.0 3.5 70

Munich malt Dark lager, various Color, flavor and fermentable sugars 79 11.8 8 45

Crystal malt Ales, light, alcohol-

reduced, various

Fuller body, aroma, color, foam retention,

flavor stability

75 12.0 50 na

Carapils Lagers, light, alcohol-

reduced

Fuller body, foam retention, flavor stability 78 12.0 12 na

Amber malt Ales, stouts Source of color and flavor 75 na 20 na

Chocolate malt Ales, stouts Source of color and flavor 75 na 450 na

Black malt Stouts Source of color and flavor 73 na 550 na

Acidic malt Lagers, wheat Reduce pH in wort 73 10.5 2.0 na

Wheat malt Wheat, various Fermentable sugars, flavor, foam retention 85 13.3 2.0 110

Diastatic malt High adjunct lagers,

grain distilling

Enzymes 76 10.9 na 160

na, not available.

Data adapted from Briggs (1998), Narziß (1999), and Anonymous (2001).

3674 MALT/Malt Types and Products

crystal malts are dried at higher temperature (150–

180



C) in order to develop colors and flavors. The

times and temperatures of the drying are adjusted to

meet the color specifications of the customer. Crystal

malts give a sweet, caramel flavor to beer. In addition,

they produce a golden-colored beer with increased

fullness and good foam retention. The significant

increase in Maillard products leads to a greater reduc-

ing power and the ability to improve flavor stability

by as much as 50%.

0023 Carapils is a crystal malt but with less color due to

the use of lower temperatures during final drying.

This malt, which is sweet but without the caramel

notes, is often added in small portions to lager-type

beers to improve body, foam retention and flavor

stability without changing the color or flavor of the

final product. Carapils is used at higher concentra-

tions in light and alcohol-free beers in order to in-

crease the body of these beers without increasing

fermentability. Crystal and carapils malts contain no

active enzyme activity.

Brown and Amber Malts

0024 Brown and amber malts are specialty dark-colored

malts that have little or no enzyme activity. They are

used in small proportions to increase beer color and

add a biscuit-like flavor to stouts and top fermented

ales. Brown malts are made by roasting low-moisture

green malts. However, in order to prevent starch con-

version, as in crystal malts, brown malts are partially

dried at low temperatures with good air flows before

roasting at temperatures of 130



C. In contrast,

amber malts begin with a pale ale-type malt that has

been previously kilned. The kilned malt is roasted at

temperatures of 100–150



C, depending on the color

desired. Amber malt is the more popular of the two

types, but demand for both types has decreased in

recent years.

Chocolate and Black Malts

0025 Chocolate and black roasted malts are highly colored

malts that give strong colors and particular flavors

to beer. They are customarily used in stouts where

they bring a flavor described as dry, burnt, astringent

but with the hint of a rich, sweet taste. The malts

start with an undermodified malt that has been

kilned at low temperatures to a moisture of 4–6%.

The dried malt is transferred to a roaster where tem-

peratures are raised to 215 or 220



C for chocolate

and black malts, respectively. The major differences

between the two malts is the length of roasting with

the darker-colored black malts requiring longer times.

In both cases, the high temperatures destroy all

enzymes.

Other Brewing Malts

0026Several other types of barley malt with specific char-

acteristics are available. Acidic malts are used to

improve beer quality by lowering the pH of wort,

especially in breweries using water with a high

carbonate hardness. During mashing, the lower pH

increases enzyme activity, especially proteases, a fact

that sometimes results in the name ‘proteolytic malt.’

Acidic malt produces a beer with a paler color and a

more mellow taste. The malt is produced by adding

lactic acid or lactic acid bacteria to the barley during

malting. Acidic malt can be used to avoid the need for

additives such as sulfuric acid.

0027Chit or short malts are used as a cheaper source

of fermentable sugars in areas where adjuncts are

illegal. These malts are produced by severely limiting

germination and kilning in order to produce an

inexpensive product. The undermodification of chit

malt, though, can lead to separation and filtration

problems in the brewery, and therefore, it is used in

limited proportions.

0028Brumalts are dark-colored and rich in Maillard

products due to a high-temperature, anaerobic rest

during the final day of germination. They are used to

bolster the malt aroma of dark German beers such as

Alt beer. Smoked malts are sometimes required in

specialty beers such as Rauch beer. The malt is pro-

duced by allowing smoke to pass through the malt

during the kilning process.

Wheat Malt

0029Wheat is the second most commonly malted cereal

grain. The malt is a major ingredient in traditional

wheat beers such as those of Belgium and Germany.

Small portions of wheat malt are also added to stand-

ard ales and lagers in order to improve foam retention

and yeast nutrition, which results from the different

types and levels of protein in wheat versus barley

malt. As a result of the missing hull, wheat malt

produces higher levels of extract and lower levels of

tannins, the latter can cause hazes in beer. A small but

significant amount of wheat malt also goes into

foods, where it produces a sweet luscious flavor.

(See Wheat: Grain Structure of Wheat and Wheat-

based Products.)

0030Wheat can be difficult to malt because the missing

hull and smaller kernel size cause kernels to take

water up more quickly. Overmodification must be

avoided as it leads to excess solubilization of protein

and the potential for hazes in the final beer. The

smaller size of wheat kernels can also lead to process-

ing problems in the malthouse, such as dense packing.

Dark wheat malts are sometimes produced for

German wheat beers, and for the food industry, by

MALT/Malt Types and Products 3675

malting wheat in a manner similar to that used for

Munich malts.

Sorghum Malt

0031 In Africa, sorghum is routinely malted on a commer-

cial basis. The malt is used predominantly to produce

traditional opaque beers such as kaffir as well as

breakfast cereals. However, with a growing demand

for European-type beers and with low barley produc-

tion and high tariffs on barley malt, sorghum malts

are also being used in Africa to make European-type

beers.

0032 It is difficult to produce a sorghum malt with ap-

propriate quality for brewing. Germinating sorghum

does not produce an abundance of enzymes, espe-

cially cell-wall-degrading enzymes. As a result, there

is poor modification of the endosperm. Exaggerated

malting conditions, such as long germination times

and elevated temperatures, tend to increase the break-

down of starch and protein rather than cell walls.

These conditions result in increased malt losses,

sometimes as high as 20%, and excess levels of sol-

uble protein in beer. Further problems are encoun-

tered in the brewery where high starch gelatinization

temperatures combined with the heat lability of

enzymes lead to incomplete breakdown of starch

and thus poor fermentability. The residual starch,

combined with high levels of b-glucan, can lead to

filtration problems. The problems of malting sor-

ghum are being addressed through the development

of new varieties with better enzyme potential and

through changes to the brewing process such as

the use of microbial enzymes and of mash filters.

(See Sorghum.)

Malt of Other Cereals

0033 Rye, triticale, and oats are the only other cereals that

are malted to any extent commercially. Malted rye is

used traditionally in the rye whiskey industry, and

some malted rye is used to add a ‘spicy’ flavor when

brewing beer. Rye malt can produce extremely high

levels of extract and enzymes, but its high levels of

nonstarch polysaccharides cause increased viscosities,

which result in problems with wort separation and

beer filtration. These problems, along with a ten-

dency to form hazes in the finished beer, have limited

the use of rye in brewing. Malted triticale has seen

limited use in the production of specialty beers, where

it produces intense flavors. Beer made from triticale

malt also has problems with high viscosity and excess

levels of soluble protein, which cause the beer hazes.

0034 The use of malted oats for brewing continues

to decrease. The thick husk of oats causes reduced

levels of extract, and a high level of lipid can lead to

off-flavors in the final beer. Low levels of malted oats

are still used in traditional oat stouts.

Malt for Distilling and Vinegar Production

0035Malt requirements differ between malt and grain

distillers who use malt or malt plus adjunct in their

mashes, respectively. Malt distillers require a malt

that produces a maximum amount of fermentable

sugars from its starch. There is no need for unfermen-

table dextrins. The malt, therefore, must have good

levels of starch and starch-degrading enzymes. The

malting conditions needed to produce this type of

malt are similar to those used for pale lager malts. A

plump barley is selected and well modified during

germination. Milder kilning conditions are used to

conserve enzyme activity. There is no need for color

or flavor development in distillers malt, which also

strengthens the use of low kilning temperatures. The

one unusual practice with this malt can be the burn-

ing of peat during kilning. Varying levels of peat

smoke can be passed through the bed during kilning

in order to impart a lightly or heavily peated flavor, as

demanded by distillers.

0036Grain distillers are primarily concerned with the

levels of starch-degrading enzymes in their malt. The

malt enzymes must degrade both the malt starch and

the adjunct starch into fermentable sugars with, once

again, no need for unfermentable dextrins. As with

adjunct brewing, grain distiller’s malt must have high

levels of amino acids in order to support yeast growth.

Maltsters meet these specifications by starting with a

thinner, higher-protein barley. The barley is well

modified and then killed at low temperatures to

conserve enzyme activity. Grain distiller’s malt is

sometimes referred to as ‘diastatic malt’ because of

extremely high levels of starch degrading enzymes

(i.e., diastatic power). Diastatic malts are also used

in brewing and in food applications when high levels

of enzyme activity are required.

0037Vinegar processors require malt of a quality similar

to that demanded by distillers. Processors that use all-

malt mashes require a malt with high levels of both

enzymes and starches, a profile similar to that

demanded by malt distillers. In contrast, vinegar pro-

cessors that use adjuncts require a malt with high

enzyme levels and an adequate supply of amino

acids, a quality profile similar to that of grain distil-

lers. Vinegar processors are not interested in the pres-

ence of dextrins or the development of flavor and

color in their malts.

Malt Extracts and Syrups

0038Malts are often processed into extracts or syrups in

order to facilitate further processing. Extracts are

3676 MALT/Malt Types and Products

made by following the first few steps of brewing. A

malt is mashed in order to complete the solubilization

and conversion of starch into fermentable sugars as

well as solubilization of protein. The resulting extract

is separated from the spent grains and concentrated

through evaporation under vacuum. Extracts destined

for brewpubs, brew-on-premise, or home brewing are

often supplemented with hops prior to concentration.

003 9 Syrups are produced in a manner similar to extracts

except that adjuncts replace some of the malt primar-

ily to reduce costs. The quality of syrups differs from

that of extracts in that reducing sugars are increased,

whereas soluble protein levels are lower, resulting in a

lighter color and a distinctly mellower and sweeter

flavor. Final levels of enzymes, flavors, and colors, in

both extracts and syrups, can be altered by starting

with different malts, by changing the mashing condi-

tions or by changing the conditions used for concen-

trating the extracts and syrups.

004 0 Dry extracts and syrups can be formed by spray

drying the liquid extracts and syrups. The color,

flavor, and sweetness are preserved, but the heat in-

activates the enzymes. Dry diastatic malts with a high

enzyme activity, therefore, are produced by mixing

milled malt with wheat flour. The final product con-

tains enzymes from the malt and potential extract

from the wheat but has a limited ability to produce

color or flavor.

Malts for Food Products

0041 Food industries require an array of malt types with

different levels of flavor, color, and enzymes. They

depend on brewing type malts that have been

custom-made to meet their specifications. Extracts,

syrups, and their dried products are often used, pro-

vided they have appropriate specifications, by the

food industry as they can simplify processing.

Seealso: Barley; Beers:HistoryandTypes;Raw

Materials;ChemistryofBrewing;Microbreweries; Bread:

DoughMixingandTestingOperations; Browning:

Nonenzymatic; Cereals:BreakfastCereals; Enzymes:

Uses in Food Processing; Malt: Chemistry of Malting;

Sorghum; Syrups; Vinegar; Wheat: Grain Structure of

Wheat and Wheat-based Products; Whisky, Whiskey,

and Bourbon: Products and Manufacture

Further Reading

Anonymous (2001) Pauls Malt Limited. http://

www.paulsmalt.co.uk.

Bemment DW (1985) Specialty malts. The Brewer 71:

457–460.

Briggs DE (1998) Malts and Malting. London: Blackie Aca-

demic & Professional.

Briggs DE, Hough JS, Stevens R and Young TW (1981)

Malting and Brewing Science: Volume 1 Malt and

Sweet Wort, 2nd edn. London: Chapman & Hall.

Finney PL (1982) Effects of germination on cereal and

legume nutrient changes and food or feed value: A com-

prehensive review. In: Nozzolillo C, Lea PJFA and Loe-

wus PA (eds) Recent Advances in Phytochemistry.

Volume 17: Mobilization of Reserves in Germination.

pp. 229–305. New York: Plenum Press.

Henry RJ and Kettlewell PS (eds) (1996) Cereal Grain

Quality. London: Chapman & Hall.

Hickenbottom JW (1996) Processing, types, and uses of

barley malt extracts and syrups. Cereal Foods World

41: 788–790.

MacGregor AW and Bhatty RS (eds) (1993) Barley Chemis-

try and Technology. St. Paul, MN: American Association

of Cereal Chemists.

Narziß L (1991) Special Malt for Greater Beer Type Variety,

pp. 284–292. Brauwelt International 9: 284–292.

Narziß L (1999) Die Technologie der Maltzbereitung, 7th

edn. Stuttgart: Ferdinand Enke.

Palmer GH (ed.) (1989) Cereal Science and Technology.

Aberdeen: Aberdeen University Press.

Piggott JR, Sharp R and Duncan REB (1989) The Science

and Technology of Whiskies. Harlow, UK: Longman

Scientific and Technical.

Simpson JP (1995) Special malts. The Brewer 81: 455–458.

Chemistry of Malting

M S Izydorczyk and M J Edney, Canadian Grain

Commission, Winnipeg, Manitoba, Canada

History

0001Beer, one of the most popular alcoholic beverages, is

made from four basic ingredients: malted grain, hops,

yeast (Saccharomyces spp.), and water. There is

evidence that as long as 4000 years ago, the Egyptians

and Mesopotamians enjoyed malted barley drinks

that were the predecessors of our modern beer. Malt

is a natural product of a process, called malting,

whereby the grain is allowed to germinate under con-

trolled conditions of moisture, temperature, and air.

Early malsters copied the natural process of germin-

ation by steeping the grain in containers and then

spreading it out in a thin layer across a floor. Once

germination was completed, the ‘green malt’ was

dried over open fire. The early maltsters were true

craftsman passing down the skills of malting by word

of mouth from generation to generation. Nowadays,

malting is a large-scale commercial process yielding

MALT/Chemistry of Malting 3677

value-added, modified grain product used primarily

for the production of beer and distilled products such

as whiskey, and, to a lesser extent, as an ingredient in

certain food stuffs. Although many cereals have been

processed into malt, barley (Hordeum vulgare)is

considered the best malting grain, so the chemical

processes described below are those occurring in

barley grain during malting.

0002 Technologically, malting is divided into three stages:

steeping, germination, and kilning. However, the com-

plex biochemical and chemical reactions occurring

during malting are interrelated and often not confined

to one stage of malting. Those reactions involve both

major and minor constituents of barley grain, which

are interconverted under the influence of various

enzymes synthesized and/or activated during malting.

The main purpose of malting, from a technological

point of view, is to render the barley kernel more sol-

uble in hot water and susceptible to fermentation by

yeast. Extensive changes that occur in the grain during

malting include complete destruction of the endosperm

cell walls, breakdown of the protein matrix, controlled

solubilization of barley proteins, and formation of

hydrolytic enzymes, which carry out degradation of

starch granules at subsequent stages of the brewing

process. (See Beers: History and Types.)

Composition of Barley Grain

0003 The husk and pericarp, the two most outer and pro-

tective tissues of the barley grain, consist primarily of

hollocellulose, hemicellulose, lignin, and lignans

(Figure 1). The testa and the nucellar cuticle, the

pigment strand, and the micropylar structures make

up the semipermeable system that divides the interior

from the exterior of the grain and limits the ingress of

water and dissolved substances. The husk and peri-

carp are not extensively altered during malting, but

they do play a role in the process by limiting the rate

of infusion of oxygen and water in respiring grain.

Moreover, the husk protects the acrospire and the

entire grain against physical damage. The husk also

has major effects on the character, flavor, and stability

of beer. Removal of husk from the grain leads to a

blander and more insipid beer, whereas grain en-

riched in husk produces astringent beers with a ten-

dency to form a haze.

0004The grain embryo contains little starch but is rich

in protein (34%), lipids (14–17%), and ash (5–10%)

and extremely rich in sugars (sucrose 5–10%; raffi-

nose, 5–10%; fructans). During germination, the

embryo synthesizes and releases gibberellin hormones

and a complex mixture of hydrolytic enzymes, which

are involved in mobilizing the reserves in the grain

and supporting the embryo’s metabolism and growth.

0005The rest of the mature grain consists of nucellar

material, aleurone and endosperm. The thick walls of

the aleurone layer (3–5 mm) contain mainly arabin-

oxylans (67–71%) and smaller amounts of b-glucan

(26%). The tissue contains three layers of cells and no

starch but abounds in proteins (17–20%), triacyl gly-

cerol (20%), minerals, phytin, and sugars (including

sucrose, raffinose, stachyose, verbascose, and fruc-

tans). Aleurone cells have bodies called aleurone

grains. These consist of globoids, which contain

phytin, and crystalloids, which contain protein and

polysaccharides. The cells of the aleurone layer, like

those of the embryo, are living. In hydrated grain,

they are capable of synthesizing and secreting a var-

iety of enzymes needed for breakdown of the reserves

stored in the starchy endosperm.

0006The starchy endosperm makes up about 75% of the

barley grain and is the major reserve tissue. Starch,

confined to small and large granules, constitutes

about 80% of the endosperm. In addition, the starchy

endosperm contains a relatively large amount of

proteins (*9% of the tissue) and smaller amounts

of lipids, minerals, and nucleic acids. The cell walls

(2 mm) are mainly built up from mixed linkage

b-(1 !3, 1 !4)-glucans (70%), arabinoxylans (20%),

and small amounts of proteins, b-(1 !3) glucans

and other polysaccharides containing galactose,

mannose, and uronic acids. In mature grain, the

cells of the starchy endosperm are nonliving and

function solely as reserves of carbohydrates, proteins,

lipids, and nucleic acids awaiting dissolution during

germination. (See Barley.)

Regulation of the Germination Process

0007Barley germination commences at relatively low

levels of grain moisture, but rapid and uniform

germination for malting purposes requires a grain

Scutellar

Epithelium

Scutellum

Acrospire

Rootlets

Coleorhiza

Micropyle

Crushed cells

Pigment

strand

Husk

Pericarp

Testa

Aleurone

Starchy endosperm

Proximal end Distal end

Embryo

fig0001 Figure 1 Longitudinal section of a mature barley grain showing

component tissues.

3678 MALT/Chemistry of Malting

moisture content of 40–45%. Most of the water that

penetrates the barley kernel does so near the embryo,

probably through the micropylar region. The bio-

chemical processes of germination also require

oxygen. The entry of oxygen into the interior of the

grain is hindered by the diffusion barriers of the husk

and pericarp, and by the water film on the surface of

the grain. The proper hydration and oxygen uptake of

barley grain for malting purposes are accomplished

by multiple submersions in water alternated with air

rest during steeping, the first stage of the malting

process.

0008 At the onset of germination, the embryo uses its

internal reserves of sugar, lipids, proteins, and min-

erals to initiate development of roots and a shoot. A

mixture of hydrolytic enzymes is released from the

scutellar epithelium into the endosperm. The gibber-

ellins, a family of plant hormones, are formed in the

germinating embryo and migrate to the aleurone

layer, where they trigger enzyme production and/or

enzyme release.

0009 The mechanisms by which gibberellins trigger the

responses of aleurone layers are not fully understood,

but there is some evidence for gibberellin receptors on

the surface of protoplasts prepared from the aleurone

layer. Exactly which gibberellins are present in

malting barley also remains unclear. The main gibber-

ellin appears to be GA

, with smaller amounts of

gibberellic acid GA

also present (Figure 2). The pro-

duction and release of gibberellins seem to be affected

by the concentration of sugars in the embryo. The

process probably involves a feedback control loop,

i.e., sugar depletion in scutellum ! gibberellin for-

mation and release ! gibberellin-triggered enzyme

synthesis and release from aleurone layer ! endo-

sperm breakdown and sugar release ! sugar diffu-

sion into scutellum ! increased sugar level in embryo

! cessation of gibberellin formation and release.

There is no clear evidence that, in germinating barley,

other nongibberellin hormones influence the forma-

tion of hydrolytic enzymes. However, some hormones

such as indolacetic acid, cytokinins, abscisic acid, and

ethylene, when added to isolated aleurone, influence

the response of the tissue. Abscisic acid, in particular,

is a potent inhibitor of gibberellin-induced enzymes

synthesis in aleurone cells.

0010The scutellar epithelium also plays a role in pro-

ducing hydrolytic enzymes. It has been suggested that

the mixture of enzymes released from the scutellar

epithelium differs from that produced by the aleurone

layer. Embryos do not need an external supply of

gibberellins to produce enzymes, but when supplied

with amino acids, their productivity increases. It is

agreed that some isoenzymes, e.g., carboxypeptidase

I, (1 !3, 1 !4)-b-glucanase I, and some a-amylases,

are generated in the scutellum. Biochemical studies

now suggest that the scutellum accounts for approxi-

mately 1–10% of a-amylase production; the remaining

a-amylase is produced by the aleurone layer.

0011The enzymes released by ‘triggered aleurone’ layers

include a-amylase, (1 !3, 1 !4)-b-glucanase, (1 !3)-

b-glucanase, proteases, peroxidase, and enzymes able to

cleave DNA, RNA, various disaccharides, glycosides,

phosphates, and peptides.

0012Although aleurone responses are primarily trig-

gered by gibberellins, it is also possible that other

substances act as modulators in this process. It has

been shown that tannins can act as gibberellin antag-

onists. Also, high concentration of sugars and other

compounds, which create a high osmotic pressure,

reduce enzymes formation by the aleurone layer.

However, calcium ions promote the secretion of

a-amylase as well as the formation of b-glucanase.

Chemical Reactions During Malting

0013The migration of hydrolytic enzymes, actively se-

creted from the aleurone and the scutellar epithelium

into the starchy endosperm, where they depolymerize

cell walls, storage proteins, starch, and residual

nucleic acids, is brought about by a rather slow

water-assisted diffusion process. Endosperm dissol-

ution begins in the region immediately adjacent to

the scutellum and progresses toward the distal end

of the grain. Before they can reach their substrates in

the starchy endosperm, the migrating enzymes must

first overcome two physical barriers: the walls of the

secretory cells themselves and the walls of starchy

endosperm cells.

Degradation of Cell-wall Polymers

0014b-Glucans The b-glucans are linear polymers com-

posed of d-glucopyranosyl residues (Glcp) linked via a

mixture of b-(1 !3) (*30%) and b-(1 !4) (*70%)

linkages. The linkage arrangement is not completely

irregular; consecutive blocks of b-(1 !4) linkages,

mostly two or three, but occasionally up to 20, are

separated at random by single b-(1 !3) linkage

(Figure 3). The b-glucans are high-molecular-weight

polymers only partially soluble in water. As no

evidence for covalent cross-linking has been found,

COOH

fig0002 Figure 2 Chemical structure of gibberellins GA

and GA

MALT/Chemistry of Malting 3679

it seems likely that less soluble b-glucans are

physically entrapped with insoluble proteins, arabi-

noxylans, or cellulose.

0015 The breakdown of partially soluble b-glucans may

have to be initiated by the release and solubilization

of these polymers from the cell walls. Enzymes, such

as carboxypeptidase, protease, phospholipase, cellu-

lase, and/or esterase may be involved in these

initiating processes. The actual depolymerization of

b-glucan occurs by the action of two major hydrolyz-

ing enzymes, endo-(1 !3, 1 !4)-b-glucanases (iso-

enzymes EI and EII; EC 3.2.1.73), which are

developed in almost equal proportions in germinating

barley (Table 1). The enzymes catalyze the hydrolysis

of b-(1 !4) linkages within the polymeric chains.

These require adjacent b-(1 !3)- and b-(1 !4)-

linked glucose residues next to the b-(1 !4) linkage

being hydrolysed (Figure 3).

0016The endo-b-glucanases produce mainly tri- and

tetrasaccharides, which may be further degraded to

glucose by exo-b-glucanase (EC 3.2.1.74). Three exo-

b-glucanases of molecular weight 67 000–70 000

have been purified from grain and shown to hydro-

lyze both the b-glucans and the oligosaccharides

released by the action of endo-(1 !3, 1 !4)-b-

glucanases to glucose.

0017During malting, the total amount of b-glucans de-

clines considerably; the content of b-glucans in barley

may vary from 3 to 6%, and after malting, it should

range from 0.2 to 1%. High levels of b-glucans in

malt reflect incomplete cell-wall degradation, affect

the yield of malt extract, and produce viscous ex-

tracts, causing filtration difficulties in the brewery

and contributing to formation of hazes and precipi-

tates in beer.

0018Arabinoxylans Arabinoxylans are side-chain bran-

ched heteroglycans built from pentose sugars, arabi-

nose and xylose. They consist of a linear xylan

backbone of b-(1 !4)-linked d-xylopyranosyl resi-

dues (Xylp) to which a-l-arabinofuranosyl units

(Araf ) are attached mostly as single substituents

(Figure 4). Some arabinose side-chains carry esterified

residues of ferulic acid. As a consequence, arabinox-

ylan chains in cell walls may be cross-linked with each

other via ferulic acid bridges and/or cross-linked with

other cell-wall polymers via ether or ester linkages.

Glc Glc

Glc Glc Glc

Glc Glc Glc Glc

Glc

Glc Glc

Glc Glc Glc

Glc Glc Glc Glc

Glc Glc

(Glc)

Glc

Glc Glc

Glc

Glc Glc

Glc

(Glc)

+ +

Endo-β-glucanases

fig0003 Figure 3 Generalized structure of barley b-glucans. Glc, D-glucopyranose residue;  and |, b-(1!4) and b-(1!3) linkages,

respectively, between the glucose residues; n ¼ 1–20; ", sites of hydrolysis by endo-b-glucanase.

tbl0001 Table 1 Properties of barley endo-b-glucanases

Property Isoenzyme EI Isoenzyme EII

Amino acids 306 306

Apparent molecular weight

(SDS-PAGE)

30 000 32 000

Isoelectric point 8.5 10.6

pH optimum 4.7 4.7

Carbohydrate content (%) Trace *4

Thermal stability Low Greater

Secretion site Scutellum, aleurone Aleurone

Data compiled from Fincher GB (1992) Cell wall metabolism in barley. In:

Shewry PR (ed.) Barley: Genetics, Biochemistry, Molecular Biology and

Biotechnology, pp. 413–437. Wallingford, UK: CAB International; and

Fincher GB and Stone BA (1993) Physiology and biochemistry of

germination in barley. In: MacGregor AW and Bhatty RS (eds) Barley

Chemistry and Technology, pp. 247–295. St. Paul, MN: American Association

of Cereal Chemists.

3680 MALT/Chemistry of Malting

0019 The routes for hydrolytic breakdown of barley

arabinoxylans are less known. Their depolymeriza-

tion to constituent monosaccharides is mediated by

the combined action of endo-(1 !4)-b-xylanases (EC

3.2.1.8), b-xylopyranosidase (EC 3.2.1.37), and a-l-

arabinofuranosidase (EC 3.2.1.55). The amount of all

these enzymes increases during malting. It is believed

that, initially, a-l-arabinofuranosidase catalyzes the

removal of arabinose residues from arabinoxylan

chains or arabinoxylan oligosaccharides. When a suf-

ficient number of arabinose substituents have been

removed, the xylan chain is hydrolyzed by (1 !4)-b-

xylanases to smaller oligosaccharides. Three endoxy-

lanase isozymes have been isolated from barley grain,

but their hydrolytic actions have not been fully investi-

gated. b-Xylopyranosidase catalyzes the hydrolysis of

xylobiose to xylose and possibly removes the unsub-

stituted xylose residues from the end of xylan chains.

As free pentose sugars do not accumulate in malting

grain, they must be rapidly utilized by the living

tissues. While b-glucans are degraded extensively

during malting, arabinoxylans are not; the majority

of barley arabinoxylans remain present in malt. (See

Carbohydrates: Classification and Properties.)

Starch Degradation

0020 The insoluble starch granules stored in the starchy

endosperm cells contain two polymers: amylose

(20–30%) and amylopectin (70–80%). Although

built up from the same glucopyranose units, these

polymers differ in the type of glycosidic linkages and

require different enzymes for their degradation. Amy-

lose consists predominantly of linear chains of about

2000–5000 of a-(1 !4) linked d-glucopyranosyl

residues. In granules, amylose may be associated

strongly with lipids, and the resulting amylose–lipid

complexes may be less susceptible to enzymic degrad-

ation. Amylopectin consists of very-high-molecular-

weight molecules, estimated to be 10

–10

Da. It

differs from amylose in that its a-(1 !4)-linked

chains of glucose are highly branched through

a-(1 !6) linkages, and therefore, amylopectin is

characterized by an extensively branched structure.

0021During malting, starch is only partially degraded

(15–18%), and the composition of the residual starch

may be slightly altered. The amylose content may

increase slightly, and the molecular weight of both

polymers is decreased. There is some evidence that

small starch granules are more susceptible to degrad-

ation than the large starch granules. Also, close to

the embryo, starch granules are almost completely

degraded, whereas those further away show evidence

of limited enzymic attack in the form of either

roughened surfaces or ‘pinholes’ (Figure 5). Most im-

portant, all enzymes necessary for total degradation of

starch, which occurs at subsequent stages of brewing,

are synthesized and/or activated during malting. In the

germinating grain, a considerable number of enzymes

are involved in the degradation of starch.

0022a-Amylases The a-amylases are able to hydrolyze

intact starch granules with the formation of soluble

products. They are responsible for the initial degrad-

ation of starch granules during malting. a-Amylases

(EC 3.2.1.1) are endoenzymes that cleave internal

a-(1 !4) glucosyl linkages of amylose and amylopectin

Xyl Xyl Xyl Xyl Xyl Xyl Xyl Xyl Xyl Xyl

Ara Ara Ara Ara Ara

Ara

fig0004 Figure 4 Generalized structure of barley arabinoxylans. Xyl,

D-xylopyranose residues; Ara, a-L-arabinofuranose residues;

# and %, sites of hydrolysis by b-xylanase and a-arabinofurano-

sidase, respectively.

(a)

(b)

fig0005Figure 5 Scanning electron micrograph of the starchy endo-

sperm showing the large and small starch granules (a) before

and (b) after germination.

MALT/Chemistry of Malting 3681

in a random fashion. They are unable to hydrolyze

the a-(1 !6) glucosyl linkages at the branch points in

amylopectin. Their hydrolytic action is slower on

short chains, and they vary in their ability to hydro-

lyze the a-(1 !4) linkages in close proximity to the

branch points. a-Amylases, acting on their own, are

able to degrade amylose to a mixture of shorter linear

a-glucan chains (linear a-dextrins), oligosaccharides,

maltose, and glucose. Amylopectin, however, is de-

graded to a lesser extent, yielding a mixture of still

large branched fragments (branched a-dextrins) as

well as some linear oligosaccharides, maltose and

glucose (Figure 6). Two isoenzymes of a-amylases

with the same apparent substrate specificities and

action patterns, but different properties, are synthe-

sized in germinating barley (Table 2).

0023b-Amylases The b-Amylases (EC 3.2.1.2) are exo-

enzymes that cleave the penultimate a-(1 !4) linkage

from the nonreducing end of the polymeric chains

and release the disaccharide maltose. b-Amylase,

acting alone, can degrade amylose completely to

maltose. This enzyme will not, however, attack the

a-(1 !6) linkages or a-(1 !4) linkages close to the a-

(1 !6) links; therefore, it degrades only the outer

chains of amylopectin and leaves a large portion

(called b-limit dextrins), in which the outer chains

have been degraded to stubs of two or three glucose

residues adjacent to the a-(1 !6) linkages (Figure 6).

b-Amylase is not synthesized during germination. In

barley, virtually all b-amylases occur in the starchy

endosperm. The four major isoenzymes occur in free

or protein-bound forms. During malting, the level of

free b-amylase may initially fall but subsequently in-

creases, as nearly all enzymes are liberated by the

action of proteolytic enzymes.

0024Limit dextrinase Limit dextrinase (EC 3.2.1.10) is

a debranching enzyme that hydrolyzes specifically

a-(1 !6) linkages in amylopectin or in branched

dextrins derived by the actions of a-orb-amylases

(Figure 6). The hydrolytic action of this enzyme

results in the formation of linear a-(1 !4)-linked

chains that can be extensively depolymerized to

glucose and maltose by the combined actions of a-

and b-amylases. The limit dextrinase in germinating

barley hydrolyzes the a-(1 !6) linkages of branched

dextrins faster than those in large amylopectin

molecules. Mature barley contains low levels of

the enzyme but during germination, the activity of

the enzyme increases due to de novo synthesis in the

aleurone. However, barley contains a heat-stable pro-

tein that inhibits the enzyme. This inhibitor decreases

in amount during malting, but there is a sufficient

amount left in malt to inhibit most of the limit

Glc

Glc Glc...

(Glc)

α-Amylase β-Amylase

Limit dextrinase

Glc

+ Glc Glc

Glc + Glc

Glc

(Glc)

Glc

Glc...

Glc

maltose

++ +

Glc

Glc Glc Glc

Glc

(Glc)

Glc

α-Limit dextrin β-Limit dextrin

Mixture of linear dextrins

Glc

Glc Glc Glc...

(Glc)

fig0006 Figure 6 Summary of the modes of action of some enzymes important in degradation of starch polymers. Glc, D-glucopyranose

residue; , a-(1 !4)-link; ", a-(1 !6)-link; NR, nonreducing end; R, reducing end.

tbl0002 Table 2 Properties of a-amylase I and II from germinating

barley

Property a-Amylase I a-Amylase II

Molecular weight 45 342 45 005

Isoelectric point 4.8–5.3 5.9–6.6

pH optimum 3.0–5.5 5.0–5.4

Stability at pH 3.6 Relatively stable Unstable

Reaction with

endogenous inhibitor

No inhibition Strong inhibition

2þ

binding Very strong Strong

Major secretion site Aleurone Aleurone

Proportion in

germinating grain

Minor (up to 10%) Major (up to 90%)

Data compiled from Briggs DE (1992) Barley germination: biochemical

changes and hormonal control. In: Shewry PR (ed.) Barley: Genetics,

Biochemistry, Molecular Biology and Biotechnology, pp. 369–401.

Wallingford, UK: CAB International; Hill RD and MacGregor AW (1988)

Cereal a-amylases in grain research and technology. In: Pomeranz Y (ed.)

Advances in Cereal Science and Technology, vol. IX, pp. 217–261. St. Paul,

MN: American Association of Cereal Chemists; and MacGregor EA and

MacGregor AW (1987) Studies of cereal a-amylase using cloned DNA. CRC

Critical Reviews in Biotechnology 5: 129–142.

Barley a-amylase/subtilisin inhibitor.

3682 MALT/Chemistry of Malting

dextrinase. Although purified limit dextrinase has no

action on starch granules, it enhances the rate of

granule dissolution when combined with the actions

of a-orb-amylases.

0025 a-Glucosidase a-Glucosidase (EC 3.2.1.20) releases

glucose residues from the nonreducing end of a var-

iety of a-glucosides and, therefore, participates in the

final conversion of maltose and other small dextrins

to glucose. After germination is initiated, rapid syn-

thesis of two or more isoenzymes of a-glucosidase is

observed in both the aleurone layer and the embryo.

(See Starch: Functional Properties.)

Chemical Changes in Proteins During Malting

0026 Proteins account for about 8–15% of the dry weight

of the mature barley grain. Albumins and globulins –

the water- and salt-soluble proteins, respectively –

function as enzymes, metabolic regulators, structural

or storage proteins, or inhibitors of particular

enzymes. Hordeins are insoluble in salt solution but

dissolve in warm aqueous alcohol, whereas the re-

sidual insoluble material constitutes the fourth group

of barley protein, glutelin. Hordeins belong to the

group of cereal proteins known as prolamines

because of their high proline and glutamine content.

They are the major storage proteins of barley. The

glutelin fraction contains storage and structural

proteins. The quantities and relative amounts of

the fractions change substantially during malting.

Following hydration, the protein reserves in the scu-

tellum and aleurone layer are degraded, and at least

some of the liberated amino acids are used to synthe-

size hydrolytic enzymes, which are released into the

starchy endosperm. The level of hydrolytic enzymes

increases during malting and declines during kilning.

The endogenous inhibitors of many enzymes decline

during malting. Reserve proteins in the endosperm

are substantially degraded to soluble peptides and

amino acids by the action of many proteolytic

enzymes. The simple peptides, amides, and amino

acids diffuse to the embryo, where they may be com-

pletely degraded, interconverted, and utilized by the

growing embryo to form new proteins in the growing

rootlets and acrospire. In a well-modified malt, 40–

45% of the total protein is water-soluble.

0027 Degradation of proteins in germinating barley is

accomplished by a range of proteolytic enzymes,

including endopeptidases (proteases) and exopeptidases

(Figure 7). Four groups of endopeptidases (EC 3.4.21),

cysteine, serine, aspartic, and metalloproteases, are

produced by germinating barley. The cysteine proteases

are dominant, accounting for up to 90% of the total

protease activity. The endopeptidases can attack pro-

teins and large polypeptides, hydrolyzing peptide

bonds from the interior of the chain and thus rapidly

diminishing the molecular weight of proteins. During

germination, protease activity in the starchy endosperm

increases several-fold.

0028Exopeptidases release one amino acid at a time

from either the carboxyl or the amino end of the

peptide chain. Five carboxypeptidases (EC 3.4.16.1)

have been identified in germinating barley. Each

enzyme attacks peptides exclusively from the

COOH-termini of the chain and has a distinct prefer-

ence for particular amino acids. However, their

specificities are complementary, and together, they

effect an extensive degradation of barley proteins. In

addition to carboxypeptidase, several other exopepti-

dases, including four neutral aminopeptidases,

leucine aminopeptidase, and dipeptidase, have been

identified in germinating barley. The aminopeptidase

and dipeptidase are active mainly in the scutellum

and are involved in the absorption of amino acids by

the growing embryo. The carboxypeptidases, how-

ever, are found both in the starchy endosperm and

in the scutellum, and can release amino acids from

hordeins as well as from smaller peptides. They might

also be involved in the solubilization of b-glucans

during the early stages of germination.

Chemical Changes in Other Grain Reserves

0029The total lipid content of barley may vary from 2 to

4.4%. Barley lipids comprise 67–78% of nonpolar

lipids (mostly triacylglycerides (TG), some steryl

esters and free fatty acids (FFF)), 8–13% of glycoli-

pids, and 14–21% of phospholipids. Lipids associ-

ated with starch granules are almost exclusively

lysophospholipids and free fatty acids. It is generally

accepted that during germination, storage lipids are

used as a source of metabolic energy and fatty acids

for synthesis of new membrane and structural lipids.

The overall decrease of total lipids by 10–50% may

occur during malting, but the changes in the compos-

ition of lipid classes are minor and/or not clearly

elucidated. The lipolytic activities in germinating

barley increase. Enzymes such as lipases and

phospholipases act in concert with lipoxygenase.

When the barley tissue is stimulated by gibberellins,

the aleurone bound lipase (EC 3.1.1.3) is activated

and responsible for hydrolyzing the ester bonds in TG

AA AA AA AA AA AA AA AA COOH

Protease Protease

Aminopeptidase Carboxypeptidase

fig0007Figure 7 Summary of the modes of action of some enzymes

important in degradation of proteins during malting.

MALT/Chemistry of Malting 3683